Variable speed technology using permanent magnet synchronous motors (PMSMs) for compressors is one of the emerging technologies to lead efficiency improvements in today's Heating, Ventilating and Air Conditioning (HVAC) systems. Different from induction motors, PMSMs need to know rotor position to be able to control the rotor. Since the motor is normally disposed inside the compressor without any speed sensor, rotor angle and speed estimation is needed to control the PMSM compressors. Most rotor angle and speed estimation techniques operate at relatively low efficiencies at low speeds due to noise and low currents, however, and the general approach in these cases is therefore to start the compressor in an open loop control scheme and then switch to a sensorless vector control scheme.
With this approach, the switch transition may at times cause high spike currents, oscillations and stability issues that tend to reduce the product's lifetime and may increase failure rates. Also, the PMSM compressor has to release its pressure difference, which means the compressor may not be able to immediately restart leading to energy stored in the compressor potentially being lost.